Time Domain Electromagnetic (TDEM) surveying is a rapidly developing area of geophysical surveying. It encompasses ground based and airborne applications. TDEM geological mapping involves equations for calculating the value of electromagnetic fields that are time dependent. Geological data is then inferred from the electromagnetic field data based on resistivity factors, in a manner that is known.
The TDEM method was originally designed for exploration of conductive ore bodies buried in resistive bedrock, but at the present time it is also used extensively in general geological mapping, in hydrogeology, in environmental investigations etc.
The method involves generating periodic magnetic field pulses penetrating below the Earth surface. Turning off this magnetic field at the end of each pulse causes an appearance of eddy currents in geological space. These currents then gradually decay and change their disposition and direction depending on electrical resistivity and geometry of geological bodies. The electromagnetic fields of these eddy currents (also called transient or secondary fields) are then measured above the Earth surface and used for mapping and future geological interpretation in a manner that is known.
The common technical means to generate magnetic field pulses is a known transmitter generally consisting of a loop of wire or a multi-turn coil connected to the output of a known electrical current pulse generator or transmitter driver. The typical size of a transmitter coil is a few meters in diameter for an airborne device and up to hundreds of meters for ground systems. Generally, the bigger the transmitter coil diameter the stronger its magnetic moment, which then results in deeper and more accurate investigations.
An additional multi-turn coil or an x-y-z coil system usually serves as a receiver or sensor for the secondary electromagnetic field. Magnetometers are also applicable for this purpose. Received signals are digitised by a known analog to digital converter (ADC) and processed and stored by computer.
The advantage of airborne TDEM systems is the speed with which ground that can be covered in geological surveying. However, there are a number of technical problems in designing airborne TDEM systems based on prior art.
The transmitted electromagnetic fields generally generate eddy currents not only in the Earth but also in the proximate metallic parts including those of the system and the aircraft body. The secondary fields of these currents behave as a noise due to typical instability of the system geometry and thermal changes in conductors. This noise impacts the survey data by generally decreasing their reliability for extrapolating geological data therefrom.
The most common way to minimise this noise is by keeping the receiver at an adequate distance from the transmitter driver. The result of this spaced apart relationship between the transmitter driver and the receiver is that the secondary fields of the eddy currents in the Earth are comparable with secondary fields of local metal parts and therefore noise level is negligible. This type of solution is used in the TDEM systems branded “GEOTEM” and “MEGATEM” of (FUGRO AIRBONE SURVEYS LTD) GEOTERREX PTY. LTD. This particular solution includes a bird towed behind a fixed-wing aircraft on a tow cable approximately 130 meters long.
Another prior art TDEM system consists of a helicopter towed system manufactured by T.H.E.M Geophysics Inc. This system uses a helium balloon to keep its sensor suspended at a distance apart from the transmitter system.
One of the disadvantages of these prior art solutions is that there is relatively poor horizontal resolution of the system due to the relatively long distance between transmitter coil and receiver sensor. Another disadvantage is difficulties of system mechanical management in start/landing and in flight manoeuvres.
Another prior art method currently used to minimise this kind of noise is to cancel the transmitter primary field localised in metal parts of the system using special coils producing in this local area a magnetic field having opposite direction to the main field of the transmitter coils. This technology is used in the AEROTEM™ branded solution of Aeroquest Ltd. in order to minimise the secondary fields in the metal parts of the transmitter electronics, which instead they locate in the towed bird. This solution requires a high level of system mechanical rigidity. In turn, it leads to heavier frame construction. The heavier frame results in a number of disadvantages. In particular the heavier frame makes transportation of the bird difficult. The production costs and fuel costs associated with manufacturing and use of the AEROTEM™ solution are also relatively high.
More importantly, because of the need for a rigid frame having a relatively significant weight, a frame with a generally smaller transmitter coil diameter is selected resulting in a lower transmitter dipole moment. This generally results in insufficient transmitter dipole moment to make deeper measurements.
Another problem with the prior art solutions is that they do not easily permit exploiting optimal system geometry, that is the receiver in the centre of the transmitter coil. A relatively large voltage is induced in the receiver coil by each of the magnetic field pulses. But this relatively high voltage in turn renders the receiver preamp saturated and therefore inoperative during system measurement time for a short period after this pulse. This is an important and necessary time for making measurements of the Earth's response.
As a result, the solution of existing systems is to place the system receiver at a distance away from the transmitter where the transmitted pulse is much lower since the strength of this field diminishes as the inverse cube of the distance. However, this then results in a departure from the optimal system geometry.
In the case of the AEROTEM™ system, the method of dealing with this large voltage pulse while maintaining optimal system geometry, i.e. receiver in centre of transmitter coil, is to place the receiver coil inside a bucking coil carrying the anti-phased transmitter current so as to cancel a large part of the voltage pulse induced in the receiver coil during the transmitter “ON TIME” while not substantially affecting the reception of the secondary field from the Earth.
This approach works well to solve the problem of this on-time voltage pulse problem, however, the process of accurately bucking this signal again mandates the rigid geometry of all parts including the receiver coil. This rigid mounting precludes the proper vibration isolation of the receiver coil thus unwanted mechanical vibration influences the receiver coil so as to induce electrical interference thereby reducing sensitivity.
Another technical problem is how to produce maximum magnetic moment in the transmitter coil using minimum weight, size and electrical power. In the above-mentioned systems a significant part of the total weight is used for the structure and power sources.
Another problem is the air drag of the bird during flight. Complicated support structures with large effective surface areas create excessive drag. This limits possible flight speed increasing survey cost.
Another limitation of the previously mentioned systems is the limitation on the maximum transmitter diameter and therefore obtainable dipole moment. A maximum diameter for these systems is generally attained relatively quickly because the rigidity criterion mandates significant weight of the structure. This stiffness factor forces this type of design to reach the maximum allowable weight for helicopter use before a desirable diameter is attained.